As of today, messages and logs of emerge do not use the system
locale anymore but default to English. The intention behind this
is to ease the work of bug wranglers and package maintainers,
who may have a hard time reading build logs in foreign
languages. This change only affects the language of messages
in emerge output, nothing else.

If you really want to have e.g. localized compiler error messages
in your builds, set LC_MESSAGES in your /etc/portage/make.conf.
Note that submitting localized build logs to the Gentoo Bugzilla
is discouraged. If maintainers are unable to translate the
necessary information from the build log, your bug may be closed
as NEEDINFO and you may be asked to re-open it submitting an
English build log [1] before any further action can be taken.